This invention relates to compounds which contain both a surface-selective functional group and a fluorescent moiety, and the use of such compounds in selectively coating certain components of a mixture of limestone ore particles to the substantial exclusion of other particles. The invention is particularly well suited for use with a mechanical apparatus for sorting ore particles, such as an electro-optical separator.
U.S. Pat. No. 3,356,211 to Mathews describes a method for concentrating ore which involves preferentially coating the desired particles with a liquid fluorescent material, subjecting the ore to electromagnetic radiation so that at least the coated portion will fluoresce, and sensing the characteristic fluorescent wavelength emitted by the irradiated particles.
U.S. Pat. No. 3,472,375 to Mathews describes an apparatus which senses the emitted fluorescent radiation from gangue or ore particles especially coated particles, and separates by selectively directing streams of a fluid to cause those fluorescent particles to be removed from the remaining quantity of undesired ore.
Successful operation of a mechanical sorting device such as that of Mathews is dependent upon the ability to selectively coat either gangue or ore particles which contain a particular mineral component, while not coating the other particles to a significant extent. The surface chemical properties of a specific ore or gangue particle depend upon the minerals which are present in that particle and, since the composition of individual particles can show a wide variation, the surface chemical properties of the particles will also vary.
To utilize a difference in surface chemical properties in the separation of ore particles, it is necessary to contact the mixture of particles with a surface-selective agent which will selectively react with certain mineral species present in the particles, due to the selectivity of the reagent in distinguishing between surface chemical properties. The reaction may be chemical, physical or a combination of those types. This process is referred to herein as "conditioning".
Methods of particle separation in which it is necessary to condition the particles include flotation separation and optical separation. In flotation separation, the particles to be separated are conditioned with a flotation agent, which coats the ore particles with which it is reactive and creates a hydrophobic mineral surface. When air bubbles are attached to this hydrophobic surface, the coated particles can be floated away from uncoated particles.
For an optical separation, the mixture of particles is conditioned with a suitable surface-selective reagent and either a coloring agent or a fluorescent material, depending upon the nature of the separation process. One example of a separation using a coloring agent is the method of U.S. patent application Ser. No. 897,947, filed Apr. 19, 1978 and titled "Method of Separating a Mixture of Ore Particles", now U.S. Pat. No. 4,235,708.
The procedure for applying a coating to ore particles for an optical separation usually involves application of the fluorescent or coloring agent in one of three forms: precipitated in an aqueous or non-aqueous slurry, dissolved in an organic conditioning reagent (which may be then dispersed in an aqueous medium prior to application), or direct application of the agent (either alone, in solution, or dispersed in an aqueous medium) after a conditioning reagent has been applied to the particles.
U.S. Pat. No. 3,346,111 to Thompson et al, which is incorporated herein by reference, describes a method for rendering asbestos contained in a host rock differentially fluorescent in relation to the rock. A fluorescent dye is precipitated to form a gelatinous slurry, into which the asbestos-containing particles are dipped. Some quantity of the suspension is entrained in exposed asbestos fibers, giving those particles which contain more asbestos a higher fluorescence than the particles with less asbestos.
U.S. patent application Ser. No. 897,740, now U.S. Pat. No. 4,208,272 filed Apr. 19, 1978 and titled "Separation of Limestone from Limestone Ore" describes various methods for selectively coating limestone particles, or gangue particles which do not contain major amounts of limestone, with a fluorescent dye. For selectively coating limestone, a carboxylic acid such as oleic acid or caprylic acid is used as the coupling agent. If it is desirable to coat the siliceous gangue particles, an aliphatic amine is used as the coupling agent. The application contemplates either combining a fluorescent dye with the coupling agent prior to conditioning the ore particles or applying a fluorescent dye to the conditioned particles. However, the preferred method is to combine the coupling agent and fluorescent dye prior to conditioning, both to realize a lower dye consumption and to simplify the process.
Examples of processes for the separation of oil shale particles may be found in U.S. Pat. No. 4,169,045 and in U.S. patent application Ser. No. 45,185, filed June 4, 1975. An example of a process for the separation of coal particles is U.S. patent application Ser. No. 897,779, filed Apr. 19, 1978, now U.S. Pat. No. 4,208,273.
U.S. Pat. No. 3,901,793 to Buchot, et al, which is incorporated herein by reference, describes a three-step process for applying a fluorescent coating to mineral particles, involving a preconditioning by washing with water plus a wetting and a scouring agent, treating with a collector ("second conditioning"), and finally applying the fluorescent reagent.
The selection of conditioning agents and coloring or fluorescing agents is of utmost importance in developing a sorting process for a particular ore, utilizing one of the previously described systems.
Excluding the disclosure of U.S. Pat. No. 3,346,111, previously noted, which is a purely mechanical entrainment of fluorescent dye suspension by asbestos fibers, the foregoing references show the use of mixtures of coupling agents (which selectively bond to the desired ore particles) with fluorescent or coloring agents or the sequential application of a coupling agent and a mutually compatible fluorescent or coloring agent. When mixtures are contemplated, they comprise either solutions or dispersions (including emulsions) of coupling agent and fluorescent or coloring agent in an aqueous or organic carrier, depending upon the nature of the components.
Ordinarily, the coupling agent and the fluorescent or coloring agent are both insoluble in water so that subsequent steps, such as rinsing to remove the weakly adhering coating from undesired ore particles, will not greatly remove the coating from desired particles. The control of washing conditions can significantly improve the selectivity of a separation. However, if it is desired to provide a dye to particles which are selectively non-coated with the coupling agent to the substantial exclusion of the coated particles, a water insoluble coupling agent and a water soluble dye, or a water soluble coupling agent and a water insoluble dye can be used.
Such contact between aqueous media and water-insoluble materials can result in the unwanted formation of emulsions. Emulsions which form can be particularly difficult to remove, since the fine ore particles which are produced during crushing to the desired size range cannot always be completely removed by pre-washing, and these particles are incorporated into the emulsions, thereby increasing the emulsion stability. This has the effect of causing difficulties in the handling of process streams and preventing the recycling of materials in the process.
U.S. Pat. No. 2,560,425 to Fancher describes the preparation of the choleretics having the formula Ar-CO--(CH.sub.2).sub.n COOH, in which Ar is either fluoranthyl or tetrahydrofluoranthyl, and n is 2 or 3. U.S. Pat. No. 2,773,091 to Burtner relates to similar derivatives of fluoranthene in which the (CH.sub.2).sub.n function is expanded to include bivalent, aliphatic hydrocarbon radicals containing up to 8 carbon atoms.
These references are examples of fluorescent molecules (fluoranthene and tetrahydrofluoranthene) which have been provided with specific functional groups (carboxy and oxocarboxy). Such compounds, and the processes for their preparation, can be useful in the practice of the present invention.